Hi,

I ask this before starting the model itself as it was a question asked of me by my boss. He is under the impression that if I can work out stresses and displacements that I can find a maximum load on the end of a crane arm just by applying forces from the RAMs. I am under the impression that I won't be able to get an accurate resultant force because the model cannot be constrained under the right conditions with no load applied at the end. He is under the impression that because I could find the resultant force at a fixture I can reverse the process.

A picture of the propessed model (obviously I would need to simplify for simulation):

I haven't attempted the model because I didn't want to waste too much time so my two follow up questions would be:

1. If it is possible how is it achieved?

2. If it is not possible how do I explain that to my boss?

Thanks for any help,

Mark

It depends.

If you load is applied slowly (ie time-independent) and a linear approximation of your system provides good accuracy, then it's possible. There could be some special caviets depending upon the specifics of the system, but generally speaking, you'd solve the system with a unit load (ie a load with a unit vector in the proper direction for the load and a magnitude of 1) and then determine the factor of safety on the highest (non-singular) stress. Your maximum allowable load is the FOS (times the load, which in this case is one, thereby making your maximum load just the FOS).

However, if the loads are highly transient (ie time-dependent) or your system has some severe non-linearities associated with it, then you need to use an iterative approach (ie solve the model with different "guesses" for the maximum load and see if it's below your failure limit).